The present invention relates to a method and apparatus for forming an extremely thin film for preventing static electricity, fogging, corrosion, mildew, etc. on the surface of magnetic tape, magnetic disks, semiconductor devices, records, plastics products, photographic film, metal products, clothing, etc.
It has been a common practice to incorporate an anti-static agent and bonding agent such as polyurethane or polyvinyl acetate into the magnetic coating for magnetic tape in order to protect the magnetic tape from static charge that causes noise and misoperation. This anti-static agent, however, adversely affects the adhesion of the magnetic coating to the tape, when used excessively.
It is also a practice to incorporate an anti-static agent directly into the tape. This method, however, is disadvantageous in that production cost is high, anti-static performance is not uniform, and the anti-static agent bleeds to the surface of the tape.
In order to eliminate static build-up that occurs on plastics molded items immediately after demolding, the molded items are dipped in or sprayed with a solution of anti-static agent so that an anti-static film is formed on the surface of the molded items. These methods encounter some disadvantages. That is, dipping requires the step of removing (drying) excess solution from the molded items after dipping. Making a uniform and firm-bonding film by dipping is difficult. The molded items might become irregular in gloss after dipping. On the other hand, spraying requires an additional step for spreading the sprayed solution, and this tends to cause static build-up under certain circumstances.
Plastics films and sheets are easy to generate static charge. Charged films or sheets cause ink to scatter during printing, and cling to the machine or become irregular during seal-cutting operation. The conventional apparatus for eliminating static charge from films and sheets is elaborate, and one that uses corona discharge encounters danger of fire due to high discharge voltage and gives off ozone which is harmful to human bodies. In addition, such an apparatus is poor in durability.
The minute irregularities on the mirror-finished surface of a plastics molded item are eliminated by forming a surface improving film on the surface. Such a film, however, does not spread uniformly along the irregularities, but tends to agglomerate in many places. This is attributable to the film thickness and the surface tension of the film greater than that of the plastics.
In the case of plastics bags, it has been a practice to incorporate an anti-static agent into the raw material or to apply an anti-static agent onto the inside of the finished bags so as to prevent the bags from becoming charged. The bags thus prepared are sticky and difficult to open, and this is a great hindrance in the case where the bags are filled with articles at high speeds using an automatic packaging machine. In addition, the anti-static agent applied to the inside of the bag adversely affects the contents of the bag. Thus this method is not suitable for bags for packing foods and drugs. Moreover, the sticky anti-static agent which has bled to the surface of the bag collects powder or particles of the contents, inhibiting the mouth from complete heat sealing. In another method for eliminating static charge, the finished bags are dipped in a solution of anti-static agent so that a layer of anti-static agent is formed on the outer surface of the bag. This method is not effective for preventing static build-up inside the bag because a continuous homogeneous layer is not formed.
Static charge is also deleterious to semiconductor devices such as IC and LSI. Preferably, in the case of MOS-type IC, the gate is broken instantaneously even by a small quantity of static charge caused by friction with other objects during transportation or by static charge accumulated in a human body. In the case of the latest 16-kilobit or 64-kilobit high-performance LSI which is designed to operate on 0.2 voltage, the performance is easily impaired and the circuits are broken by static electricity. Static charge harmful to semiconductor devices is generated when they are rubbed on the plastics magazine stick used as the transportation case. (The magazine stick used to be made of metal. It is earthed during use. Metal magazine sticks are inconvenient because the semiconductor devices in them are not visible from outside. For this reason, they have been replaced by plastics ones.) In addition, the fact that semiconductor devices themselves are insulated and coated with plastics helps generate static electricity through friction. For instance, ceramic DIP's packed in a plastics tube generate a static charge of 700 V; ceramic DIP's arranged on a plastics tray generate a static charge of 4000 V; ceramic DIP's packaged in foamed polystyrene generate a static charge of 5000 V; IC's deprived of "air cap" (cushioning material) generate a static charge of 20,000 V; IC's packed in foamed plastics generate a static charge of 11,000 V; and IS's taken out of the packing material for repair generate a static charge of 6000 V. Countermeasures for preventing static build-up are taken for semiconductor devices in the respective stages of production, storage, transportation, mounting on the apparatus, and operation of the apparatus. Such countermeasures include earthing, bonding, humidification, use of conductive materials, use of anti-static agents, use of static eliminators, wearing of anti-static working clothes and shoes, use of conductive floors, and suppression of discharge. These countermeasures have to be taken in each stage of production, storage, and transportation, and their effect is not necessarily complete. As another countermeasure for static charge, the protective container of the semiconductor device is made conductive. But this does not completely prevent static damage of semiconductor devices because instantaneous discharge and inductive charge are induced by the conductivity.